Traditionally, control signal lines for railway signal lamps and switch points have been driven from a power supply via the contacts of an electromagnetic control relay. Such relays have been developed over a number of years using special techniques and materials so that these relays presently in use are fail-safe in their operation. The occurrence of failures has been reduced to an extremely low level and the fail-safe design of the relays makes it entirely certain that a relay will fail in a predetermined manner which is always arranged to be the most restrictive condition or safe state.
However, the ever rising costs of production and materials relative to those involved in producing semiconductive devices have now made it desirable to seek ways, wherever possible, to replace relays with the cheaper solid-state devices. The problem which arises is that a semiconductive device per se cannot be made to fail to a predetermined state and hence are not inherently fail-safe. Therefore, electronic circuits have to be designed as a whole to adopt a predetermined state upon a semiconductive device failing.
A drawback in the context of railway signal interlocking systems is that long, sometimes extremely long, periods of time elapse in which railway light and switch point control signals have to remain in one or another state. In these circumstances, the problem which arises is that if a semiconductive device is in a saturated state, and fails such that it would in its failed condition, then also occupying that state, its failure will not be detected until the interlocking system next tries to change the state of that signal.